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Paris, 20 September 2010

Decoding the genome of Chlorella microalgae, a promising genus for biofuel production

The analysis of the complete genome of Chlorella microalgae, a promising genus for biofuel production, has been completed by the Laboratoire Information Génomique et Structurale of CNRS, which is currently heading an international collaboration involving American and Japanese laboratories (1). The detailed elucidation of the Chlorella genome, also widely used as a food supplement, will make it possible to rationalize its industrial use. This analysis has also brought to light unexpected findings at the fundamental level: it suggests that Chlorella could have a sexual cycle (which had gone unnoticed so far) and that a virus probably gave it the capacity to synthesize chitin-rich cell walls (2), a unique property in algae. This work is published online on The Plant Cell journal's website.

Microalgae are prime targets for research on biofuels. Leading candidates as alternative sources of biodiesel, their culture has the unquestionable advantage, compared to oleaginous land plants, of not competing with cultivated land necessary for human food. Producing fuel from water, sunlight and carbon dioxide from the atmosphere appears as a miracle solution that has fostered numerous research programs since the 1970s.

Chlorella is particularly interesting for the development of second-generation biodiesel thanks to its high lipid content (it only contains 30 % dry matter). Although several genomes of green algae (Chlorophyta) have already been sequenced (Chlamydomonas, Micromonas and Ostreococcus), Chlorella had not been analyzed until now, despite its economic role as long-established food supplement. The analysis of the Chlorella genome, coordinated by Guillaume Blanc, CNRS researcher, predicts 9,791 protein genes, a total comparable to that of its cousin Micromonas. This new genomic data will help to further rationalize the use of Chlorella in various industrial processes.

The comparative analysis of the different genomes of green algae has made it possible to paint a genetic portrait of their common ancestor, which seems to have already possessed most of the phytohormone biosynthesis pathways necessary to the development and growth of land plants.

In an unexpected fashion, the analysis of the Chlorella genome has also revealed numerous genes governing the synthesis of flagellar proteins, which suggests that this species could have a sexual cycle that has gone unnoticed until now. Last but not least, the ability of Chlorella algae to synthesize chitin could have been inherited from a virus (itself endowed with chitinase activity (3)) having secured exclusive use of its host against other viruses incapable of piercing through its protective shell. This “monopoly” scenario illustrates a new mode of co-evolution between viruses and their hosts.

A Chlorella algae “lawn” on a Petri dish, showing small circular “patches” (small lighter circles), revealing the presence of a virus

Notes:

(1) The Joint Genome Institute, Department of Energy, (Walnut Creek, California), the University of Nebraska (Lincoln, Nebraska), the Georgia Institute of Technology (Atlanta, Georgia), the City University of New-York (Brooklyn) and the University of Hiroshima (Japan).
(2) One of the main components of the exoskeleton of insects and other arthropods and of the cell wall of some species of fungi. This substance had never before been encountered in the green lineage (plants).
(3) Chitinase is an enzyme capable of specifically breaking down chitin.